Ring extruder feed

ABSTRACT

The invention relates to a multi-screw extruder for the continuous treatment and/or processing of a bulk material, especially a product in powder, granular or flake form, comprising several shafts ( 3 ) which are arranged in a ring shape in a cavity ( 1 ) of an extruder housing ( 2 ) and which extend parallel to the axial direction (A) of the extruder. The extruder housing ( 2 ) is provided with axially parallel, concave segments ( 5   a   , 6   a ) of a circle on the radially inner ( 5 ) and radially outer ( 6 ) surfaces of the cavity ( 1 ). These circle segments act as a guide for the axially parallel shafts ( 3 ) with their processing elements ( 4 ) on the inside ( 5 ) or on the outside ( 6 ) of the shaft ring ( 3 , . . . ). The inventive improvement lies in the radial extension of the axial partial area of the cavity ( 1 ) containing the shafts ( 3 ), which is located in the area of the feed opening ( 9 ), this radial extension ( 10   b ) extending along part of the shaft ring ( 3 , . . . ) in the peripheral direction (U) of the same.

FIELD OF TECHNOLOGY

This invention relates to a multi-screw extruder for continuouslymachining and/or molding bulk material.

BACKGROUND

Multi-screw extruder technology has established itself in recent years.Primarily extruders with several screws arranged in a ring or circle,which all each mesh with two adjacent screws, e.g., the 12-screwRingExtruder®, are characterized by particularly high throughputs andnarrow retention time spectra. Bulk materials with a high apparentdensity, e.g., granules, are particularly advantageous for molding.

However, when the objective is to mold loose bulk material with arelatively low apparent density (approx. 20 to 60% of solids density),e.g., flocs or macerate, in such a multi-screw extruder, the problembecomes that the extruder feed opening is only able to draw in bulkmaterial arriving in a loose bed with a high content of air with a lowthroughput at the feed opening. Means common in prior art, such asstuffing screws at the feed opening and/or degassing the extruder casingdirectly on the side opposite the conveying part of the feed opening,provide only an unsatisfactory solution. Therefore, these multi-screwextruders, which in themselves offer a very high throughput, are always“underfed” in the case of loose bulk material, e.g., polyethyleneterephthalate (PET) flocs, which stem primarily from recycled bottles(RPET). Their feed opening is operationally limited for such loose bulkmaterial.

SUMMARY

Therefore, the object of the invention in a multi-screw extrudermentioned at the outset with screws arranged in a ring or circle is toimprove the feed performance in such a way that the possible throughputis approximately reached even given relatively loose bulk material, andan underfeeding of the extruder is largely avoided.

This object is achieved by the multi-screw extruder according to oneembodiment of the present disclosure.

The radial expansion in the area of the feed port of the extruder allowsmost flocculent or macerated particles of the bulk material, inparticular bottle RPET, to more readily get to the conveying and feedingworm elements in the feed zone, so that the worm elements can betterseize and draw them in. In this way, far more flocs or macerate per unitof time can be drawn into the extruder.

The radial dilatation of the expansion at the feed port is preferablymaximal, and narrows starting from the peripheral location on the screwcircle where the feed port is located, along the periphery of the circleto a non-expanded peripheral location on the screw ring, where theconventional clearance is present between the screws of the screw ringand casing. In this way, a “feed pocket” is created outside and/orinside the circle between the screw circle and the radially externalsurface of the extruder cavity or between the screw ring and theradially internal surface of the extruder cavity, whose conveying crosssection diminishes in the feeding process as the macerates/flocs becomeincreasingly compressed, which brings about a considerable rise in thefeed performance for loose bulk material.

Particularly advantageous is an extruder according to the invention thathas screws rotating in the same direction, and in which the screw ringhas a through opening at least in the area of the feed port between theinternal ring section of the cavity and the external ring section of thecavity, and the expansion extends in the peripheral direction of thescrew ring away from the feed port on both sides, wherein a firstportion of the expansion extends between the radially internal side ofthe cavity and the screw ring (inner feed pocket), and a second portionof the expansion extends between the radially external side of thecavity and the screw ring (outer feed pocket), so that the surface ofthe machining elements of the identically rotating screws extending intothe respective portion of the expansion during operation of the extrudermoves in the narrowing direction of respective expansion. This designmakes it possible to achieve a particularly high feed performance.

Instead of a relatively large through opening between the internal ringcavity and the external ring cavity, e.g., which is formed by omittingan entire screw at least in the feed zone, it may be sufficient to makethe through opening out of sections of adjacent worm elements that donot tightly intermesh, at least in the area of the expansion.

As a result, at least bulk material compressed and melted furtheropposite the conveying side of the feed opening within the extruder canget from the radially external section of the cavity into the radiallyinternal section of the cavity, thereby also increasing the fill levelof the extruder.

It is also advantageous if the expansion along the periphery of thescrew ring extends from the feed port on either side, and extendsbetween the radially external side of the cavity and the screw ring(bilateral external pocket). Even though one of the two external pocketsdraws in less strongly than the other feed pocket with the “right” screwrotational direction only due to the gravity and, possibly, “stuffingforce” acting on the bulk material and owing to the “false” rotationaldirection of the worm elements, it also makes a positive contribution tothe overall feed performance, even if to a lesser extent.

In a particularly advantageous further development of the multi-screwextruder according to the invention, the radial dilatation of theexpansion in the area of the feed port is maximal, and this expansionnarrows from the axial location where the feed port is located, alongthe axial conveying direction of the extruder, up to a non-expandedaxial location, where the conventional clearance between the screw ringand casing is again present. The at least one feed pocket is narrowednot just in the peripheral direction, but also in the axial conveyingdirection of the extruder. Since the worm elements act to draw in notjust in the peripheral direction when seizing the flocs/macerate, butalso convey, and hence feed, in the axial direction, this design makesit possible to achieve an optimal feed performance.

In particular, the radial expansion narrowing in the axial conveyingdirection consists of several individual segment expansions, which areeach allocated to one of the circular segments, wherein the segmentexpansions are preferably designed in such a way that the narrowed areasbetween or next to the respective segment expansions are not expanded,and the individual segment expansions are largely separated from eachother.

The segment expansions are best designed in such a way that theprojection of the outside surface of the respective segment expansionhas roughly the form of a sickle on a plane perpendicular to the axialdirection, wherein the sickle surfaces consist of the differentialsurfaces between the outlines of the expanded extruder “flower” at theone axial location, and the outlines of the non-expanded extruder“flower” at the other axial location.

The segment expansions, i.e., their outside surfaces, are preferablydesigned in such a way that their largest radial expansion (relative tothe midpoint of the respective screw in the segment expansion) extendsin a radial direction twisted by an angle φ in the rotational directionof the screws relative to the radial direction R determined by themidpoint M of the extruder and the midpoint m of the respective screw.This angle φ lies between roughly 20° and 60°, preferably roughlybetween 30° and 50°. This angular shift causes the “leaves” of therespective “flower” (section of the outside surfaces of the segmentexpansions with a plane perpendicular to the axial direction) to be torninto “tears”. This significantly increases the feed performance. It alsomakes sense for at least some of the narrowed areas to have an expandednarrowing in the peripheral area of the expansion.

All configurations specified above can be advantageously combined with astuffing screw adapted to the multi-screw extruder and/or with ventholes provided at the extruder casing near the feed port, wherein apressure under atmospheric pressure is preferably applied to the feedport.

BRIEF DESCRIPTION OF THE DRAWINGS

Additional advantages, features and possible applications of theinvention can now be gleaned from the following description of preferredembodiments of the invention based on the drawing, which are not to beregarded as limiting. Shown on:

FIG. 1 is a cross section through a multi-screw extruder from prior art,perpendicular to its longitudinal axis;

FIG. 2 is a cross section through the multi-screw extruder of FIG. 1 inthe area of its feed opening according to prior art;

FIG. 3 is a cross-section analogous to FIG. 2 through a multi-screwextruder with a feed opening according to the first embodiment;

FIG. 4 is a cross-section analogous to FIG. 2 through a multi-screwextruder with a feed opening according to the second embodiment;

FIG. 5 is a cross-section analogous to FIG. 2 through a multi-screwextruder with a feed opening according to the third embodiment;

FIGS. 6A, 6B and 6C are different perspective views of the casingsection of the feed zone of the multi-screw extruder according to theinvention based on a fourth embodiment; and

FIG. 6D is a view of the casing section from the feed zone of themulti-screw extruder according to the invention based on the fourthembodiment, as seen in the conveying direction of the extruder.

FIG. 1 is a cross section perpendicular to the axial conveying directionthrough a multi-screw extruder from prior art. This extruder has tenscrews 3, which are arranged in a circle, and form a screw ring 3, . . .Each screw 3 carries with it rotationally fixed machining elements 4,e.g., worm elements or kneading elements (not shown). The screw ring 3,. . . is situated in a cavity 1 of the extruder casing 2. Both theradially inner lying surface 5 and the radially outer lying surface 6 ofthe cavity 1 contain recesses designed as cylinder jackets, which extendin the axial conveying direction of the extruder, and whose crosssection resembles an inner lying circular segment 5 a and an outer lyingcircular segment 6 a. These recesses or circular segments 5 a are usedas a guide for the screws 3 provided with machining elements 4. Thecavity 1 is divided by the screw ring 3, . . . into a inner-ring segmentla and an outer-ring segment 1 b.

FIG. 2 is a cross section through the multi-screw extruder from priorart from FIG. 1 in the area of its feed opening. The same referencenumbers on FIG. 2 and FIG. 1 each relate to identical or correspondingelements. The feed port 9 extends in the peripheral direction over fiveof the total of ten screws 3, and in the axial direction (perpendicularto the drawing plane) typically over a stretch of about 1 to 3 screwwindings, depending on the pitch of the windings. All screws 3 rotate inthe same direction as denoted by the arrow D.

If a bulk material is now supplied to the extruder via the feed port 9,it is drawn into the extruder cavity 1 adjacent in the axial directionby the rotating screws 3 with their worm elements (see FIG. 1). In bulkmaterial with a high apparent density, e.g., grainy materials, the feedopening works satisfactorily. However, if the bulk material to be drawnin involves one of low apparent density, e.g., flocculent or porousmaterial, whose flocs are still intertwined, this feed geometry of amulti-screw extruder from prior art will be able to draw in only a veryinadequate quantity of material lying far below the throughput enabledby the cavity 1 and the number of screws 3. This “underfeeding” of theextruder arises in particular when macerates of recycled polyethyleneterephthalate bottles (RPET) are recycled, and is especially disruptive,since only a fraction of the possible throughput can be processed.

FIG. 3 is a cross-section analogous to FIG. 2 through the multi-screwextruder according to the invention based on a first embodiment. Theidentical reference numbers on FIG. 3 and preceding figures each relateto the same or corresponding elements. The feed port 9 is here expandedon one of its sides along the peripheral direction U, as opposed toprior art. This expansion 10 essentially consists of a section 10 blying radially outside the screw ring 3, . . . , which extendsproceeding from a peripheral location U1 at the feed port along theperipheral direction U around nearly the entire screw ring 3, . . . upto a peripheral location U2 at which practically no more expansion ispresent. The expansion 10 b is narrowed along the peripheral direction,i.e., its radial expansion ΔR tapers with increasing peripheral positionbetween U1 and U2. This makes it possible to increase the feedperformance of the extruder for the aforementioned flocculent ormacerate bulk material, and satisfactory fill levels and throughputs arealso achieved for such materials. In order to fill the internal ringsection 1 a of the cavity in addition to the external ring section 1 bof the cavity 1 (see FIG. 1), the screws 3 that engage each other due totheir machining elements 4 are not closely intermeshed. A pressure Punder atmospheric pressure is preferably applied to the feed port 9. Astuffing screw 8 adapted to the multi-screw extruder is also shown inFIG. 3.

FIG. 4 is also a cross-section analogous to FIG. 2 through themulti-screw extruder according to the invention based on a secondembodiment. The identical reference numbers on FIG. 4 and precedingfigures again relate to respectively the same or corresponding elements.At least in the feed zone, where a screw is omitted, the feed port 9 ishere connected with the internal ring side section la of the cavity 1,which is expanded along the peripheral direction U. The expansion 10here essentially consists of a section 10 a lying radially inside thescrew ring 3, . . . , which also extends proceeding from a peripherallocation U1 at the feed port along the peripheral direction U aroundnearly the entire screw ring 3, . . . up to a peripheral location U2 atwhich practically no more expansion is present. The expansion 10 a isalso narrowed along the peripheral direction, i.e., its radial expansionΔR tapers with increasing peripheral position between U1 and U2. Thisdesign of the expansion also makes it possible to increase the feedperformance of the extruder for the aforementioned flocculent ormacerate bulk material, and in order to fill the internal ring section 1a of the cavity in addition to the external ring section 1 b of thecavity 1 (see FIG. 1), the screws 3 that engage each other due to theirmachining elements 4 are not closely intermeshed.

FIG. 5 is also a cross-section analogous to FIG. 2 through themulti-screw extruder according to the invention based on a thirdembodiment. The identical reference numbers on FIG. 5 and precedingfigures again relate to respectively the same or corresponding elements.The external ring expansion 10 b is designed as in the first embodiment,while the internal ring section 1 a of the cavity 1 is additionallyexpanded to a feed pocket 10 a, but one that has a constant radialexpansion over the entire periphery U, as opposed to the feed pocket 10a of the second embodiment. The ten screws 3 of the screw ring 3, . . .are designed not to be closely intermeshed in the sections 9 b, 9 c, 9d, 9 e and 9 f, so that bulk material drawn in via the feed pocket 10 bcan gradually also get into the internal ring section 1 a of the cavityafter comminuted and/or melted.

In both the first, second and third embodiments, the partial sections 10a and/or 10 b of the expansion 10 form a feed pocket, into which theloose bulk material to be introduced is drawn in due to the force ofgravity, and primarily by the rotation D of the screws 3 accompanied byincreasing compression. In addition to the “main feed zones” 10 a or 10b, the “secondary feed zone” 10 c contributes to the overall feedperformance. The feed pockets 10 a, 10 b and 10 c also have a narrowing(not shown) in the axial conveying direction (perpendicular to plane ofprojection). This also helps to increase feed performance.

FIGS. 6A, 6B, 6C and 6D show different views of the casing segment fromthe feed zone of the multi-screw extruder according to the inventionbased on a fourth embodiment of the invention. The core of the extruderand the screws are not shown. The identical reference numbers as in thepreceding figures again relate to respectively the same or correspondingelements. As opposed to the first, second and third embodiments, theextruder of the fourth embodiment has twelve screws (not shown).

FIG. 6A shows a perspective view of the casing segment 2 for the feedzone of the 12-screw extruder. In the area of the “main feed pocket” 10a and “secondary feed pocket” 10 c, more or less material is removedfrom the outer surface 6 of the cavity, so that a larger and a smallerfeed pocket 10 a, 10 c are formed together with the screw ring (notshown). Except for two circular segments 6 a of the outside surface 6 ofthe cavity, all other circular segments are “leveled”. Vent holes V₁, V₂are provided at the extruder casing 2 near the feed port 9.

FIG. 6B shows another perspective view of the casing segment 2 for thefeed zone of the 12-screw extruder. In the area of the “main feedpocket” 10 a and “secondary feed pocket” 10 c, more or less material isremoved from the outer surface 6 of the cavity, so that a larger and asmaller feed pocket 10 a, 10 c are formed together with the screw ring(not shown). As best visible from FIG. 6B, wedge expansions 11 a, 11 band 11 c are also provided in the axial direction A in addition to theexpansions 10 a and 10 c in the peripheral direction U. This yields anarrowing of the corresponding circular segments of the outside surface6, which extends between the axial location A1 and the axial locationA2.

FIG. 6C is another perspective view of the casing segment 2 for the feedzone of the 12-screw extruder, and FIG. 6D presents an orthogonal viewas seen in the extruder conveying direction A.

FIG. 6D shows the casing segment 2 in such a way that its “flower”formed by the edge lines of the numerous circular segments (“leaves”)become clearly discernible. More precisely stated, the casing segment 2has an expanded flower B1 (the edge line lying to the front on FIG. 6D)and an unexpanded flower B2 (the edge line lying to the back on FIG.6D), between which the outer segment surfaces 14 a, 14 b, . . . , 14 i,14 j of the segment expansions 12 a, 12 b, . . . , 12 j are located. Thewedge zones 13 a, 13 b, . . . , 13 j, 13 k by or between the segmentexpansions are not expanded in this configuration. The differentialsurface between the front flower B1 and the back flower B2 represents asickle-shaped projection S of the outer segment surfaces in the area ofthe segment expansions.

To improve the feed performance, the segment expansions 12 a, 12 b, . .. , 12 j are each designed in such a way that their greatest radialexpansion Δr, relative to the midpoint m of the respective screw 3,extends in a radial direction r twisted by an angle φ in the rotationaldirection D of the screws 3 relative to the radial direction Rdetermined by the midpoint M of the extruder and the midpoint m of therespective screw 3. As a result, the anterior flower B1 appears as acircle of “tears”, or the projected “moon sickles” S appear somewhatdistorted.

REFERENCE LIST

-   1 Cavity-   1 a Internal ring section of the cavity-   1 b External ring section of the cavity-   2 Extruder casing-   3 Screw-   3, . . . Screw ring-   4 Machining element-   5 Inner lying surface of the cavity-   5 a Circular segment of the internal surface of the cavity-   6 Outer lying surface of the cavity-   6 a Circular segment of the external surface of the cavity-   9 Feed port-   9 a Through hole-   9 b Non-tightly intermeshed section-   9 c Non-tightly intermeshed section-   9 d Non-tightly intermeshed section-   9 e Non-tightly intermeshed section-   9 f Non-tightly intermeshed section-   10 Expansion-   10 a Internal section of expansion-   10 b External section of expansion-   11 a Wedge expansion-   11 b Wedge expansion-   11 c Wedge expansion-   12 a Segment expansion-   12 b Segment expansion-   12 c Segment expansion-   12 d Segment expansion-   12 e Segment expansion-   12 f Segment expansion-   12 g Segment expansion-   12 h Segment expansion-   12 i Segment expansion-   12 j Segment expansion-   13 a Wedge zone-   13 b Wedge zone-   13 c Wedge zone-   13 d Wedge zone-   13 e Wedge zone-   13 f Wedge zone-   13 g Wedge zone-   13 h Wedge zone-   13 i Wedge zone-   13 j Wedge zone-   13 k Wedge zone-   14 a External segment surface-   14 b External segment surface-   14 c External segment surface-   14 d External segment surface-   14 e External segment surface-   14 f External segment surface-   14 g External segment surface-   14 h External segment surface-   14 i External segment surface-   14 j External segment surface-   A Axial direction-   U Peripheral direction-   ΔR Radial dilatation of expansion-   U1 Peripheral location at feed port-   U2 Peripheral location without expansion-   A1 Axial location at feed port-   A2 Axial location without expansion-   B1 Expanded flower-   B2 Unexpanded flower-   D Rotational direction of screws-   ΔR Radial dilatation of expansion-   Δr Radial dilatation of segment expansion-   M Midpoint of extruder-   m Midpoint of screw-   φ Angular direction of dilatation Δr-   S Sickle-shaped projection

1. A multi-screw extruder for continuously machining and/or molding bulkmaterial, the extruder comprising: an extruder casing including: acavity, the cavity including a feed port proximate a first axial end ofthe cavity, and an outlet proximate a second axial end of the cavity;and radially inner and radially outer lying surfaces of the cavity andaxially parallel, concave circular segments; and a plurality of screwsrunning parallel to an axial direction of the extruder, the plurality ofscrews arranged in a screw ring inside a radially expanded axial area ofthe cavity, the radially expanded axial area being proximate the feedport, the radially expanded axial area extending along a portion of thescrew ring in a peripheral direction of the screw ring, and narrowingfrom the first axial area where the feed port is situated, along theaxial direction of the extruder, to an unexpanded axial area, theaxially parallel screws guided by the concave circular segments, themachining elements of the screws proximate an inside or outside of thescrew ring; wherein each of the screws carries a number of axiallyconsecutive machining elements, the machining elements intermeshing withadjacent screws, a portion of the machining elements being conveyingelements.
 2. The multi-screw extruder according to claim 1, wherein aradial dilatation of the expansion is at maximum proximate the feedport, and narrows proceeding from the peripheral portion of the screwring where the feed port is located, along the periphery of the ring upto an unexpanded peripheral location on the screw ring.
 3. Themulti-screw extruder according to claim 2, wherein the extruder hasscrews rotating in the same direction, and the screw ring has a throughhole proximate the feed port between an inner ring section of the cavityand an outer ring section of the cavity, and the expansion extends inthe peripheral direction of the screw ring on either side away from thefeed port wherein a first section of the expansion extends between theradially internal surface of the cavity and the screw ring, and a secondsection of the expansion extends between the radially external surfaceof the cavity and the screw ring so that the surface of the machiningelements of the screws rotating in the same direction that extend intothe respective section of the expansion move in the direction in whichthe respective expansion narrows during extruder operation.
 4. Themulti-screw extruder according to claim 3, wherein the through hole isformed by sections of adjacent machining elements that do not tightlyintermesh, at least in the area of the expansion.
 5. The multi-screwextruder according to claim 2, wherein the expansion extends along theperiphery of the screw ring on either side away from the feed port, andextends between the radially outer surface of the cavity and the screwring.
 6. The multi-screw extruder according to claim 1, wherein theradial dilatation of the expansion is at maximum proximate the feedport.
 7. The multi-screw extruder according to claim 6, wherein theradial expansion narrowing in the axial direction includes severalindividual segment expansions, each individual segment expansion beingallocated to one of the circular segments.
 8. The multi-screw extruderaccording to claim 7, wherein the segment expansions are configured sothat wedge zones are not expanded between or next to the respectivesegment expansions.
 9. The multi-screw extruder according to claim 8,wherein the segment expansions are configured so that the projection ofthe outer surface of the respective segment expansion has roughly theshape of a sickle in a plane perpendicular to the axial direction. 10.The multi-screw extruder according to claim 8, wherein the segmentexpansions are each configured so that their greatest radial expansionrelative to the relative to the midpoint m of the respective screwextends in a radial direction twisted by a first angle in the rotationaldirection of the screws relative to the radial direction determined bythe midpoint of the extruder and the midpoint m of the respective screw.11. The multi-screw extruder according to claim 10, wherein the firstangle lies roughly between 20° and 60°.
 12. The multi-screw extruderaccording to claim 11, wherein the first angle lies roughly between 30°and 50°.
 13. The multi-screw extruder according to claim 8, wherein atleast some of the wedge zones have a wedge expansion in the peripheralarea of the expansion.
 14. The multi-screw extruder according to claim8, wherein a stuffing screw is secured to the feed port.
 15. Themulti-screw extruder according to claim 8, wherein a pressure belowatmospheric pressure can be applied to the feed port.
 16. Themulti-screw extruder according to claim 1, wherein the extruder casinghas ventilation holes near the feed port.